None of the boss reward items are ivoryable, even the ones that say they are.

From Django: Known issues. All of the items except the robe should be ivoryable I believe. There`s been a bug in on that for a little bit, we just couldn`t fix it because of how close we were to the patch when it was opened. Robe isn`t ivoryable because it`s worth quite a large amount of experience. You can turn it in to get experience yourself from that robe if you don`t want to hold onto it and you can get it again once every day as well if you`d like.